1. Field of the Invention
The present invention relates to a method of forming a film by continuously applying a film-forming treatment to a wire or strip material (hereinafter a material to be treated or being treated is referred to as "strip material") and an apparatus thereof. In particular, it relates to a method of simply and inexpensively forming a film having a desired thickness by immersing the strip material in a solution bath containing a film-forming component and then drying it, and an apparatus thereof.
2. Description of the Related Art
In a treatment of continuously drawing a steel wire with the use of a lubricant for dry drawing, generally, a borax (borax:Na.sub.2 B.sub.4 O.sub.7) film is previously formed on a steel wire supplied from a pay off unit, and thereafter a drawing process is carried out through a series of drawing dies with the use of the lubricant for dry drawing.
The role of the borax film in drawing process is to carry the lubricant and also to prevent the die and the steel wire from directly touching each other. The thickness of the borax film is decreased with progression of the drawing process by falling off from the steel wire or elongation during drawing. In the case of continuous drawing process with a series of drawing dies, a certain thickness of the borax film on the steel wire is required even at the final die to prevent the die and the steel wire from directly touching each other. Therefore, it is necessary to form a borax film having a predetermined thickness prior to drawing, considering the reduction in film thickness during the drawing process. That is, in the case of drawing with a series of drawing dies, a thicker borax film is required when the total drawing amount is increased with a larger reduction in the section area of strip material per die and/or larger number of dies.
In such a film-forming treatment, first, strip material is immersed in a solution bath containing a film-forming component and taken out of the solution bath, and thereafter the solution remained on the strip material is drained by blowing or suction of a gas. In this case, the remaining solution should be finally drained to an extent that the remaining solution does not adhere to a jig such as a guide and the like which touches the wire before drying (hereinafter draining to this extent is referred to as "completion of draining").
The control of thickness of the film is mainly carried out by control of concentration of the film-forming component and the temperature of the solution bath. The concentration of the film-forming component in the solution bath cannot exceed the saturation limit, and also the temperature cannot exceed the boiling point, so that the amount of borax adhered on strip material, e.g. a wire material being continuously supplied and running, is limited. That is, the obtainable amount of drawing is limited because the film thickness cannot be made thicker than a limited value.
As a measure for solving such a problem that drawability is degraded due to the reduction in thickness of the borax film by progression of drawing, JP,7-195116A discloses a method of re-forming a borax film at an intermediate position of a series of drawing dies.
However, this method disclosed in the above publication has such a problem that additional units for heating and film-forming are required to be equipped at an intermediate position of a series of drawing dies. In any case, there has been a limit in thickness of a film on strip material formed by one continuous film-forming treatment without adding extra units such as a heating unit or a film-forming unit.